Diesel engines emit particulate matter such as soot, which is harmful to the environment. In order to decrease these emissions, catalytic filters are used in the exhaust pipe. However, such catalytic filters are not capable of combusting soot, since this takes place at a temperature range of 550-600° C. and therefore higher than the typical operating temperature of an exhaust (around 200° C., up to 580° C. during a combustion event)—Catalysis Today 118 2006. Catalysts capable of reducing the soot combustion temperature are known, such catalysts typically containing potassium, lithium, sodium or cesium. However, these are only capable of reducing the soot oxidation temperature to around 400° C.—Ruiz et al. applied catal. A 392 (2011)45. Catalysts containing nitrates are also known to decrease the soot oxidation temperature. However, such catalysts are not selective for CO2, i.e. they are not capable of oxidising CO, even after the incorporation of transition metals.
Gold nanoparticles exhibit catalytic activity even at room temperature, particularly when the nanoparticles are smaller than about 3 nm. However, one of the problems associated with the incorporation of gold nanoparticles into a useful catalyst is that the gold nanoparticles tend to aggregate to form larger particles, and thereby lose their catalytic activity.
It is known to use stabilising agents to reduce aggregation of gold nanoparticles. However, since stabilising agents coat the surface of nanoparticles, the catalytic activity of the nanoparticles could be reduced. However, in the case of high temperature oxidations such as, for example, those carried out at around 300° C., the stabilising agent will be destroyed.
An alternative approach to prevent aggregation of gold nanoparticles is to immobilise the gold nanoparticles on a substrate. WO2010031890A1 discloses a method in which gold nanoparticles are immobilised on carbon nanotube substrates. In order to adhere the nanoparticles to the nanotubes, the nanotubes are coated with a compound containing an amino group. The amino groups act as nucleation centres where the Au nanoparticles are stabilised, reduced and anchored in the presence of a reducing agent such as sodium citrate. This is necessary since the polymer alone cannot reduce the gold. The resulting product may be used as a dispersed catalyst or as a coating for a support surface. However, the catalytic activity of the gold nanoparticles is reduced as the stabilization and further reduction of the nanoparticles is achieved via a chemical methodology. It should be noted that the main drawback of these procedures for obtaining the catalysts is that the stabilizing agent covers the surface of the nanoclusters, and thus significantly inhibits the catalytic activity thereof.
Mendoza et al, J. Am. Chem. Soc. 2001, 133, 10251-10261, discloses a method of obtaining stable carbon nanotube-supported gold nanoclusters. The carbon nanotubes are first wrapped with the polyelectrolyte polyallylamine hydrochloride, and are then re-suspended in water at pH 9. HAuCl4 is added to the solution to form gold nanoclusters, which attach themselves to the carbon nanotubes. Sodium citrate is used as a stabiliser and soft reducing agent. The presence of citrate on the gold nanoparticles could reduce their catalytic activity.